An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as a steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism, and assist-controls the steering mechanism of the vehicle. In order to accurately generate the assist torque, such a conventional electric power steering apparatus (EPS) performs a feed-back control of a motor current. The feed-back control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a steering shaft (a column shaft or a handle shaft) 2 connected to a handle (a steering wheel) 1 is connected to steered wheels 8L and 8R through reduction gears (a worm gear and a worm) 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the torsion bar is interposed within the steering shaft 2, the steering shaft 2 is provided with a steering angle sensor 14 as the vehicle steering angle detecting apparatus for detecting a steering angle θ of the handle 1 by means of a torsional angle of the torsion bar and a torque sensor 10 for detecting a steering torque Th, and a motor 20 for assisting the steering torque of the handle 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist control command on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control command value Vref obtained by performing compensation or the like to the current command value. It is possible to receive the vehicle speed Vel from a controller area network (CAN) or the like.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
The control unit 30 mainly comprises a CPU (Central Processing Unit) (including an MPU (Micro Processor Unit) and an MCU (Micro Controller Unit)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
The control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31 which calculates the current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1, based on the steering torque Th and the vehicle speed Vel with reference to an assist map or the like, which is a control target value of a current supplied to the motor 20. The calculated current command value Iref1 is inputted into a current limiting section 33 via an adding section 32A, and the current command value Irefm whose maximum current is limited is inputted into a subtracting section 32B. A deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im which is fed-back is calculated at the subtracting section 32B, and the deviation I is inputted into a current control section 35 which performs a proportional-integral-control (PI-control) and the like for improving a current characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved at the current control section 35, is inputted into a PWM-control section 36, and the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed-back to the subtracting section 32B. The inverter 37 is constituted by a bridge circuit of field-effect transistors (FETs) as a semiconductor switching device.
A rotational sensor 21 such as a resolver, as the vehicle steering angle detecting apparatus, is connected to the motor 20 and a motor rotational angle θ is outputted.
A compensation signal CM from a compensation signal generating section 34 is added at the adding section 32A. A characteristic compensation of the steering system is performed by adding the compensation signal CM, and a convergence, an inertia characteristic and the like are improved. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 to an inertia 342 at an adding section 344. The adding result is further added with a convergence 341 at an adding section 345. The adding result at the adding section 345 is treated as the compensation signal CM.
In the electric power steering apparatus as described above, recently, the torque sensors and the angle sensors can be equipped with multiplexing due to requirements of a reliability improvement, a functional redundancy and the like. However, since the requirement of a cost reduction is also existed, it is not easy to simply multiplex the sensors. Therefore, by utilizing the currently mounted sensors at the maximum, a method that the sensors monitor and diagnose each other is preferred. Furthermore, the steering shaft of the electric power steering apparatus is connected to the motor shaft via the reduction mechanism such as the worm gear and the worm.
In a case of multiplexing the angle sensors, in other words, in a case that the dual-system angle sensors are equipped with the steering shaft and the motor shaft, when one system is failed, it is considered that the other system backs up the failed system. However, in general, since the mechanism system including the reduction mechanism and the steering system have the nonlinear elements such as friction, backlash, an elastic coupling of the motor output shaft, preload to a gear surface by means of a worm wheel and the worm, and lubricating grease of the reduction mechanism section, an angle of the steering shaft is different from that of the motor shaft and therefore an angle error occurs. In this connection, when one of the angle sensors is failed, the other of the angle sensors cannot immediately back up (substitution in the abnormality) the failed angle sensors.
As a prior art, WO 04/022414 (Patent Document 1) discloses a method for measuring a torque for a vehicle having an electromechanical steering system, and the disclosed method is considered as a torque sensor for backup. An overall configuration is an electromechanical steering system comprising an input shaft section and an output shaft section being connected to a driving steering mechanism, and a steering means having a servo motor being connected via a torsion bar. Although the configuration is the electromechanical steering apparatus (a digital circuit or an analog circuit) which performs torque detection due to a relative rotational displacement between the input shaft section and the output shaft section of the driving steering mechanism, the above apparatus forms a sensor for detecting a virtual torque by two inputs being an output of a steering angle (δ) sensor and a rotational angle of the servo motor, and the steering torque is determined from the virtual torque.
Further, in Japanese Unexamined Patent Publication No. 2005-274484 A (Patent Document 2), the apparatus is equipped with the plural steering angle sensors (three sensors) which constitute a redundant system.